Use of silver surfaces to promote dropwise condensation



Dec. 6, 1966 R. A. ERB ETAL 3,289,754

USE OF SILVER SURFACES TO PROMOTE DROPWISE CONDENSATION Filed Nov. 2, 1,964

WATER ZONE WATER VAPOR ZONE //vv/v TOR$ ROBE/PT .4. [RB EDMUND THELE/V RICHARD D. T HARKE gyw u ATTORNEY United States Patent C) 3,289,754 USE OF SILVER SURFACES TO PROMOTE DROPWISE CONDENSATlGN Robert A. Erb, Valley Forge, and Edmund Thelen and Richard D. Varlker, Philadelphia, Pa., assignors to the United States of America as represented by the Secretary of the Interior Filed Nov. 2, 1964, Ser. No. 408,433 Claims. (Cl. 165-433) The invention herein described and claimed arose out of Contract Number 14-01-0001-293 with the Ofiice of Saline Water in the Department of the Interior, entered into pursuant to the Saline Water Act, Public Law 87295.

This invention is concerned with promotion of dropwise formation of water on the surfaces of heat transfer devices, particularly condensers.

Advantages of dropwise condensation over film-type condensation, particularly in decreased thermal resistance between vapors to be condensed and the cooling fluid, is well known in the prior art, e.g., US. Patent No. 2,248,909. This patent discloses the use of zirconium and tantalum to promote dropwise condensation. Other materials have also been employed for this purpose, erg, chromium (US. Patent 2,259,024), oleic acid (US. Patent 2,919,115), resins such as polyfiuoroethylene (U.S. Patent 2,923,640) and copper sulfide (Trans. A.I.Ch.E., vol 31, No. 4, December 1935, pages 593-621).

It has now been found that very effective dropwise condensation on condenser surfaces may be achieved by the use of silver as the surface on which the water condenses. This silver surface may be achieved by the use of solid silver or by coating another metal with silver.

The invention will be more fully understood when reference is made to the accompanying drawing wherein a heat transfer device according to the present invention is shown promoting the dropwise condensation of water vapor (steam). In that drawin a water zone 29 at a temperature T is in contact with an impervious partition comprising a substrate 22, which may be a Cu-Ni alloy or steel, and a silver coating 14 which is in contact with a water vapor zone 19 having a temperature of T The temperatures of the zones are regulated so that T is less than T thereby inducing condensation on the silver surface 14 which occurs in the form of drops 12.

Silver coatings are usually achieved by means of electroplating. A base coat of nickel is usually advisable to enable the use of smaller amounts of silver. Any application technique from which a final smooth surface can 3,289,754 Patented Dec. 6, 1966 be obtained may be employed. This could include, in addition to electrodeposition, vacuum deposition (including sputtering), chemical deposition, mechanical cladding or flame spraying.

Although distilled water was used in the examples below, the method of the invention is also beneficial in promoting dropwise condensation of vapor distilled from water containing impurities, e.-g., sea water.

The invention will be more specifically illustrated by means of the following examples.

Example 1 The heat transfer apparatus used in this example consisted of a copper block having cooling water flowing through a hollowed out central portion. The block was suspended over boiling water in a heated vessel under atmospheric pressure. Attached to the block were 42 test 1 X 3 inch metal flats consisting of various test materials on different substrates, having a total thickness of about 0.05 inch. The block and flats were arranged so that the flats were disposed vertically over the boiling water. Steam arising from the boiling water condensed on the metal flats, which were in thermal transfer relationship with the cooled copper block. The water which condensed on the test flats dripped back into the boiling water.

Silver samples tested and results are given in Table l. The silver coatings were achieved by electrodeposition. The dr-opwise quality, Q, defined by the following equation, has been found to give a reliable measure of the ability of a surface to promote dropwise condensation.

Q=l.0 times the percent sample area which is dropwise condensing +0.5 times the percent mixed area. Drop appearance in the table is based on the following scale:

As is apparent from the data of the table, the silver surfaces exhibit dro-pwise condensation even after long periods of exposure to condensation.

TABLE 1.-CONDENSATION BEHAVIOR AS A FUNCTION OF TIME 1.5 Hrs. 1 Day 6 Days 10 Days 14 Days 17 Days 22 Days Drop Drop Drop Drop Drop Drop Drop Sample Type Q Appear- Q Appear- Q Appear- Q Appear- Q Appear- Q Appear- Q Appearance ance ance ance ance ance ance 500 inch Ag on 9010 Cu- 100 G+ 95 F 97. 5 G 95 G- 100 G 97. 5 F+ 97. 5 F+ Ni alloy base. 500 inch Ag on mild steel 85 97. 5 G-- 87.5 F-l- 87. 5 G- 95 G 95 G+- 97. 5 (Br-{- base. Control: 90-10 Cu-Ni 20 0 5 10 5 O 20 (polished).

28 Days 35 Days 42 Days 56 Days Days 84 Days 98 Days Drop Drop Drop Drop Drop Drop Drop Q, Appear- Q Appear- Q, Appear- Q Appear- Q Appear- Q, Appear- Q Appearance ance ance ance anee ance ance 500 inch Ag on 90-10 On- 97. 5 G- 97. 5 F 97. 5 F 97. 5 F-l- 95 F+ 95 F+ 92. 5 F

Ni alloy base. 500 ,uillCh Ag on mild steel 90 G- 95 G 85 G-l- 85 G (3+ 90 G 90 G+ base. Control: 90-10 Cu-Ni 10 5O 40 25 35 32. 5 45 (polished).

3 Example 2 The heat transfer apparatus used in this example consisted of a pressure vessel containing distilled water in the bottom thereof and having 8 condensing tubes attached to the topplate by means of a compression fitting.

The condensing tubes were /2 inch in diameter and the condensing surfaces extended 5 inches into the pressure vessel. They were cooled by means of cooling water that flowed into a central tube concentric with the outer condensing surface, to the bottom of the condensing tube, then up in the annulus between the tubes to an outlet tube above the top plate of the pressure vessel. Inlet and outlet cooling water temperatures as well as temperature of the steam within the pressure vessel were measured by means of thermocouples.

The water in the pressure vessel was heated to produce steam at 115 C. by means of three 2000 watt silver-plated immersion heaters mounted in the bottom of the pressure vessel. Self-flushing, graduated drop-catchers were also provided within the pressure vessel for measurement of condensation rate of the water on the condensing tubes. Four electrically-heated observation windows were spaced at 90 intervals around the pressure vessel.

Results are given in Tables 2 and 3. The significance of Q and drop appearance are the same as described in Example 1. The advantages of the silver-plated surfaces in condensation characteristics and condensation rate are readily apparent from Tables 2 and 3. The superiority of silver over the chromium of the prior art is particularly noteworthy.

The invention may be used in any heat transfer device in which heat is removed from a vapor through a heat transferring wall by means of a suitable cooling fluid, either liquid or gaseous, the vapor being condensed on the heat transferring wall. Examples of such devices are heat exchangers, evaporators, condensers, tubular heaters, tempering coils, etc.

What is claimed is:

1. In a heat transfer device comprising a liquid zone, a water vapor zone having a temperature greater than said liquid zone, means to conduct Water vapor to said water vapor zone, and an impermeable partition disposed between said zones whereby mass transfer between said zones is prevented, said partition having a first surface in contact with said liquid zone and a second surface in contact with said Water vapor zone, the improvement comprising utilizing silver as said second surface.

2. The device of claim 1 wherein said partition is metallic and said second surface comprises a silver coating.

3. The device of claim 2 wherein the partition is a Ni-Cu metal alloy.

4. The device of claim 2 wherein the partition is steel.

5. The device of claim 1 wherein the liquid zone contains water.

TABLE 2.-CONDENSATION BEHAVIOR AS A FUNCTION OF TIME WITH VERTICAL TUBE CONDENSERS 7 Days 21 Days 59 Days Sample Type Drop Drop p Q Appear- Q Appear- Q Appearance 31100 31169 500 pinch Ag on 500 inch Ni on -10 Cu-Ni base E- 100 G+ 100 E 90-10 Cu-Ni control 30 F 10 F 2. 5 P 500 inch Or on 1,000 pinch Ni on 90-10 Cu-Ni base 2. 5 F 2. 5 F 1. 5 P+ 500 ,uinch Ag on Type 316 stainless steel base 100 E 100 E 100 E Type 316 stainless steel controL. 2. 5 P+ 2. 5 P+ 1 P TABLE 3.CONDENSATION RATE FOR TUBE ooNDEN s References Clted by the Examlllel' g gs l xg i n n r o u i gs EXPOSURE TO CONDENSATIO UNITED STATES PATENTS 50 1,001,669 8/1911 Monnot 29199 gpq qt Water 2,259,024 10/ 1941 Cleveland 133 X Condensing Surface Y a 2,548,092 4/1951 Bartlett et a1. 1 5-433 X 6 ft./sec. 10 ft./see. FOREIGN PATENTS 568,595 4/1945 Great Britain, 5 0 inch Ag over 500 inch Ni over 90-10 96 C 83 MEYER PERLIN, Primary Examiner.

ou-Ni 0. 4s 0. 56 90-10 Ou-Ni Connor 0. 64. 0.71 ROBERT A. O LEARY, Exammer. 500 inch Ag over 316 Stainlesssteel 0. 67 0. 73 316 stainless steel control 0.41 0.46 N. R. Assrstanr Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,289,754 December 6, 1966 Robert A. Erb et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the sheet of drawing, the lower right-hand corner thereof, for "Richard D. Tharker" read Richard D. Varker Signed and sealed this 26th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD]. BRENNER Commissioner of Patents 

1. IN A HEAT TRANSFER DEVICE COMPRISING A LIQUID ZONE, A WATER VAPOR ZONE HAVING A TEMPERATURE GREATER THAN SAID LIQUID ZONE, MEANS TO CONDUCT WATER VAPOR TO SAID WATER VAPOR ZONE, AND AN IMPERMEABLE PARTITION DISPOSED BETWEEN SAID ZONES WHEREBY MASS TRANSFER BETWEEN SAID ZONES IS PREVENTED, SAID PARTITION HAVING A FIRST SURFACE IN CONTACT WITH SAID LIQUID ZONE AN D A SECOND SURFACE IN CONTACT WITH SAID WATER VAPOR ZONE, THE IMPROVEMENT COMPRISING UTILIZING SILVER AS SAID SECOND SURFACE. 